Using spectral information to aid in quantitative X-ray radiography has been widely applied since dual-energy body scans were introduced to help distinguish between bone and tissue. More recently, dual-energy radiography has been used in security inspections in an attempt to detect drugs, explosives, and high-atomic-numbered (high-Z) materials. However, a key limitation with the dual-energy technique is that it can only determine two unknowns. These scans are typically done with only two X-ray spectra since models for the components of the X-ray attenuation coefficients are calculated as functions of only density and atomic number. Dual-energy radiography can be used to determine an average density and atomic number (Z) for a given X-ray pathlength between the source and detector plane. This is a significant limiting factor for detection of nuclear materials because higher-Z materials can be shielded by lower-Z materials so that the average Z is below some threshold.
Hence, there is not currently a means for effective and automatic detection of materials, such as dense, high-Z nuclear materials, utilizing X-ray radiography, especially in multi-layered object configurations.